1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) display and, more particularly, to a circuit and method for driving pixels in an organic electroluminescent display that reduces the number of wirings of a compensation circuit for solving the brightness non-uniformity resulting from a threshold voltage difference of driving transistors arranged in an EL panel, thereby simplifying both the wirings of the EL panel and the driving method.
2. Description of the Related Art
An organic EL device is an emissive device that emits a fluorescent material by recombining an electron and a hole, with which an EL display can have a fast response time and a low driving voltage, and can be formed in a ultra-thin film, compared with a passive type light-emitting device, so that it can be applied to a wall mount type or a portable type of displays.
As a method of driving such an organic EL light-emitting cell, there are a passive matrix type and an active matrix type that uses a thin film transistor. The passive matrix type perpendicularly forms an anode and a cathode and selects a line to drive it, while the active matrix type connects a thin film transistor and a capacitor to each ITO pixel electrode to maintain a voltage with the capacitance of the capacitor.
FIG. 1 is a schematic plan view of a conventional active matrix type organic EL display having an EL panel 10, a pixel circuit 11, a scan driver 20, and a data driver 30.
The scan driver 20 sequentially outputs a selection signal through scan lines S1, S2, S3, S4, . . . , Sz, and the data driver 30 outputs a data voltage representing an image signal through data lines D1, D2, D3, . . . , Dy. The pixel circuit 11 is used to display a single pixel.
As shown in FIG. 1, the EL panel 10 includes a plurality of data lines D1, D2, D3, . . . , Dy branched from the data driver 30 to transmit the image signal, and a plurality of scan lines S1, S2, S3, . . . , Sz arranged such that a plurality of data lines and a plurality of scan lines intersect (i.e., cross over) each other. The scan lines S1, S2, S3, . . . , Sz transmit the selection signal. A pixel circuit 11 is placed at each intersection between the scan lines and the data lines.
FIG. 2 is a detailed circuit diagram illustrating the pixel circuit 11 of FIG. 1. The pixel circuit 11 includes a first thin film transistor M1, a capacitor Cst, a second thin film transistor M2, and an organic EL device OLED (e.g., an organic light emitting diode). In FIG. 2, Vdata indicates a data line in which a pixel signal is transmitted and Select indicates a scan line to which a selection signal is applied.
The data line Vdata transmits an image signal, and the scan line Select transmits a selection signal. The second thin film transistor M2 transmits data to the capacitor Cst according to the selection signal of the scan line Select, and the capacitor Cst stores and holds the applied data. Further, the first thin film transistor M1 drives the organic EL device OLED.
As shown in FIG. 2, the organic EL device OLED is supplied with a current for emitting light, by the first thin film transistor M1 connected to its anode. A cathode of the organic EL device OLED is connect to a voltage Vss (e.g., a ground voltage). Further, for the first thin film transistor M1, a source is connected to a power supply line Vdd, and a gate is connected to a drain of the second thin film transistor M2. The capacitor Cst is connected between the gate of the first thin film transistor M1 and the power supply line Vdd. Further, for the second thin film transistor M2, a gate is connected to the scan line Select, and a source is connected to the data line Vdata.
An operation of the pixel circuit having the above configuration will be described. When the second thin film transistor M2 is turned on by the selection signal Select applied to the gate of the second thin film transistor M2, the data voltage Vdata is applied to the gate of the first thin film transistor M1 through the data line Vdata. Further, corresponding to the data voltage Vdata applied to the gate, a current flows through the first thin film transistor M1 to the organic EL device OLED to emit light. Here, a voltage Vgs between the source and the gate of the first thin film transistor M1 is a difference between a voltage of the power supply line Vdd and the data voltage transmitted through the second thin film transistor M2, and the first thin film transistor outputs a current corresponding to a square of a difference between the source-gate voltage Vgs and a threshold voltage Vth of the transistor to the organic EL device. This can be represented as the following equation:IOLED=(β/2)(Vgs−Vth)2=(β/2)(Vdd−Vdata−Vth)2   (Equation 1),
where IOLED is a current flowing through the organic EL device, Vgs is a voltage between the source and the gate of the transistor M1, Vth is the threshold voltage of the first thin film transistor M1, Vdata is the data voltage, and β is a coefficient value.
As shown in Equation 1, in the pixel circuit illustrated in FIG. 2, a current corresponding to the applied data voltage Vdata is supplied to the organic EL device OLED, and the organic EL device OLED emits light corresponding to the supplied current.
The driving voltage of each power supply line Vdd varies depending on the number of turned-on first thin film transistors M1 that are connected to the power supply line Vdd. This leads to differences between the driving voltages of the connected pixels. Further, even if the voltages are the same, the difference of the threshold voltage Vth in the thin film transistor is generated due to the non-uniformity of the manufacturing process, resulting in a variance to the amount of current supplied to the organic EL device OLED, such that brightness becomes non-uniform.
FIG. 3 shows another pixel circuit, which has been designed to address the above problems associated with the conventional pixel of FIG. 2. By way of example, the pixel circuit of FIG. 3 is capable of preventing brightness non-uniformity due to the change of the threshold voltage Vth of the first thin film transistor M1. FIG. 4 shows a driving timing diagram for driving the circuit of FIG. 3.
As shown in FIG. 3, for a first thin film transistor M3, a source is connected to a driving power supply Vdd and a drain is connected to an organic EL device OLED, and for a fourth thin film transistor M6 connected between the first thin film transistor M3 and the organic EL device OLED, a gate is connected to a light emitting control line AZB. Further, a gate of the first thin film transistor M3 is connected to a first capacitor C1 and a second capacitor C2. In addition, the first capacitor C1 is connected between a source of a third thin film transistor M5 and the power supply line Vdd.
Further, for a second thin film transistor M4, a gate is connected to the scan line Select, a source is connected to the data line Vdata, and a drain is connected to the second capacitor C2. Further, for the third thin film transistor M5, a gate is connected to a threshold voltage compensation control line AZ, a source is connected between the first thin film transistor M3 and the second capacitor C2, and a drain is connected between the drain of the first thin film transistor M3 and the source of the fourth thin film transistor M6.
The conventional pixel driving circuit of FIG. 3 operates according to the timing diagram shown in FIG. 4. It can be described as follows: first, the scan line Select outputs a low signal during a certain time period to turn on the second thin film transistor M4, and the threshold voltage compensation control line AZ applies a low signal during the selected time period of the scan line, thereby turning on the third thin film transistor M5 to proceed with initialization.
Therefore, the first thin film transistor M3 serves as a diode for the driving power supply, and the second capacitor C2 stores a voltage corresponding to the threshold voltage Vth of the first thin film transistor M3.
Further, after the time period for outputting the low signal in the threshold voltage compensation control line AZ, the data voltage is charged to the first capacitor C1 through the second thin film transistor M4 as the data line Vdata applies the low signal.
Further, while the threshold voltage compensation control line AZ applies the low signal, the light emitting control line AZB turns off the fourth thin film transistor M6 by outputting a high signal until the data voltage compensates the difference of the threshold voltage Vth, thereby cutting off the driving current to the organic EL device OLED. Subsequently, when the light emitting control line AZB is changed to a low signal after a certain time period, the fourth thin film transistor M6 is turned on so that the corresponding current emits the organic EL device OLED.
However, in the conventional pixel driving circuit of FIG. 3, an additional threshold voltage compensation control line AZ and light emitting control line AZB are added in addition to the data line and the scan line, so that the wiring of the EL panel becomes more complex, and thus, the number of manufacturing processes increases, leading to an increase in the manufacture cost.